The present invention relates to a photoelectric transducer element and, more particularly, to a photoelectric transducer element having a high switching speed.
A conventional photoelectric transducer element, a phototransistor, is illustrated in FIGS. 1 and 2 or FIGS. 3 and 4.
In the phototransistor shown in FIGS. 1 and 2, a p conductivity type diffusion region 2 is formed in an N conductivity type silicon substrate 1. An N conductivity type diffusion region 3 is formed in the P-type diffusion region 2. The P-type region 2 serves as a light-receiving region (i.e., as a base region), and the N-type region 3 serves as an emitter region.
In the phototransistor shown in FIGS. 3 and 4, a base region comprises two regions having different impurity concentrations and different depths. More specifically, the base region is constituted by a P-type peripheral region 2B and a P.sup.+ -type region 2A formed continuously with respect to the peripheral region 2B. The P-type peripheral region 2B has an impurity concentration lower than that of the P.sup.+ -type region 2A, but has a depth greater than that of the P.sup.+ -type region 2A. An N conductivity type diffusion region 3, which serves as an emitter region in the same manner as in FIGS. 1 and 2, is formed in the P-type base region 2B. In the phototransistors shown in FIGS. 1 to 4, collector electrodes 5 are formed on the N-type silicon substrates 1, and base electrodes 6 are formed on the base regions, respectively. Emitter electrodes 7 are formed on the N-type emitter regions 3. Reference numeral 8 denotes an SiO.sub.2 insulation film. Here, it should be noted that the insulation film 8 is omitted in FIGS. 1 and 3.
In the phototransistors shown in FIGS. 1 to 4, since the base regions serving as light-receiving regions are wide, the number of carriers generated upon the incidence of light onto the respective base regions is great. Therefore, these phototransistors have high light sensitivity.
However, when the base region area is increased, to increase the number of carriers generated upon light incidence onto the light-receiving region, it takes time to recombine the carriers after the incident light is cut off, so that the turn-off time is increased (i.e., the switching speed is decreased), resulting in inconvenience. For example, the phototransistors shown in FIGS. 1 and 4 have a minimum turn-off time of between 15 .mu.s and 20 .mu.s. To increase the switching speed, the base area is decreased, to decrease the resistivity of the silicon substrate, so that the turn-off time is shortened and, hence, the switching speed is increased. However, the photosensitivity is thereby decreased, the dielectric withstand voltage is decreased and other problems are presented.